Children with MAS and controls will undergo standard pin-prick skin testing procedures with dilutions of histamine, codeine, and a saline control solution to assess skin wheal and flare response in order to objectively assess hypersensitivity and/or responsiveness. Skin testing specifically involves placing a drop of histamine solution on the skin surface. A needle is then penetrated gently into the epidermis. The same procedure will be used for codeine and saline. Tests will be spaced at least 2 cm apart to avoid false reactions. Testing will include two standard sites for skin testing, i.e. arm and back, for all participants. An area of skin with café-au-lait formation and its normal skin counterpart will be tested on the patients with MAS. The normal controls will undergo skin testing at a matched area. Testing will be performed in the Allergy Clinic by a trained allergy fellow or attending. Wheal and flare responses will be documented 20 minutes after the skin tests are administered. The wheal and flare response will be outlined in ink, and the image will be transferred to hypoallergenic transparent tape and placed on a data collection sheet for measurement. The longest and orthogonal erythema diameters are measured and summed. Sum of wheal diameters will be determined in a similar manner. Previous studies have shown that a standard wheal and flare response yields a sum of 7.0 cm ± 1.25 cm.